In the night sky, stars are the most conspicuous celestial objects. They appear as points of light that twinkle when they are low in the sky but shine steadily when they are high overhead. The ancients believed that the stars were gods or supernatural beings and often attributed special powers to them. In modern times, we know that stars are huge balls of gas that produce their own energy by nuclear fusion reactions.
There are two main types of stars: population I and population II. Population I stars, also called young stars, are found in the spiral arms of galaxies and have a high abundance of heavy elements. Population II stars, also called old stars, are found in the globular clusters that surround galaxies and have a low abundance of heavy elements. Most of the stars in our Milky Way Galaxy are population II stars.
Stars vary greatly in size, mass, brightness, temperature, and other properties. The smallest known star is TRAPPIST-1 which has a diameter only slightly larger than Earth’s Moon (about 1/8th as large as our Sun). The largest known star is VY Canis Majoris which has a diameter about 2200 times larger than our Sun! Stars also vary widely in mass with some being only slightly more massive than Jupiter (the largest planet in our Solar System) while others weigh several times more than our Sun. The brightest star visible from Earth is Sirius which is about 25 times brighter than our Sun; however there are many otherstars much brighter than Sirius including some which cannot be seen by the unaided eye because they lie too far away from us. Finally, stellar temperatures can range from less cool red dwarfstars like TRAPPIST-1 with effective temperatures around 2500 K to extremely hot blue supergiantstars like Rigel with effective temperatures exceeding 20 000 K!
The study of stars is called astronomy while a person who studies them is called an astronomer. Astronomers use telescopes and other instruments to collect data about stellar properties such as position, brightness, temperature, mass, and chemical composition. This data helps us understand how different types of stars form and evolve over time